The mitogen-activated protein kinases (MAPKs) integrate multiple intracellular signals transmitted by various second messengers in a complex signal transduction mechanism. MAPKs phosphorylate and regulate the activity of a variety of enzymes and transcription factors. The activity of MAPKs is triggered by a series of cascades that result in the phosphorylation of the MAPK on both a threonine and tyrosine by a MAPK kinase (MAPKK). The MAPKK is activated by a MAPKKK that becomes active by phosphorylation on its serine/threonine.
The MAPK phosphorylation cascade is highly conserved in eukaryotes. Indeed, homologs in yeast, Drosophila, mammalian cells and plants have been identified. As of 2002, more than 60 MAPKKK genes have been identified in Arabidopsis alone. (Ichimura, et al., (2002) Trends Plant Sci 7:301-308). Because of the large number of proteins involved in the cascade, it is not apparent which proteins are essential, cause lethality if absent, or are functionally redundant.
MAPKKKs and their targets have been implicated in eukaryotic organisms' growth and development. For example, in plants, MAPKKK cascades have been associated with embryonic development, cell division, disease defense responses and abiotic stress responses (Tena, et al., (2001) Curr Opin Plant Biol 4:392-400.).
It has recently been discovered that loss of function mutations in a MAPKKK gene called YODA (YDA) produce Arabidopsis plant embryos lacking a suspensor, a tissue that functions to provide nutrients from the endosperm to the growing embryo. Not all yda plants developed into mature plants and those that did exhibited delayed root development and were smaller than wild-type plants. Known plant hormones were not able to rescue the yda phenotype, suggesting a novel developmental signaling pathway. (Lukowitz, et al., (2004) Sci. STKE 2004 tw21).
Several MAPKKKs in the Arabidopsis ANP family have been identified and implicated in regulating cell division. (Krysan, et al., (2002) Plant Cell 14:1109-1120). A MAPKKK has also been identified in N. benthamiana leaves and found to play a role in the hypersensitive response and resistance against Pseudomonas syringae. (Pozo, et al., (2004) The EMBO Journal 23:3072-3082). The same MAPKKK was found to regulate cell death in susceptible leaves undergoing P. syringae infection. (Pozo, et al., (2004) The EMBO Journal 23:3072-3082).
Transgenic tobacco lines expressing different levels of the constitutively active Arabidopsis ortholog of Tobacco NPK1 were found to grow more vigorously than did the wild type plants in the presence of elevated salt levels, cold temperatures and heat shock, but phenotypically did not differ from wild type plants under normal growth conditions (U.S. Pat. No. 6,613,959). The manipulation of this oxidative stress signaling regulator can protect plant cells from diverse environmental stresses, such as heat and high salt. See, U.S. Pat. No. 6,613,959 (Kovtun, et al., (2000) Proc. Natl. Acad. Sci. USA 97:2940-2945).
Thus, MAPKKKs are involved in a variety of aspects of plant growth and development. Given the important role of members of the MAPKKK signal transduction cascade, in particular the MAPKKK signal transduction molecules, in regulating plant cellular processes ranging from cellular proliferation and differentiation to cellular apoptosis, there exists a need for identifying plant MAPKKK polynucleotides and polypeptides as well as for modulators of such molecules for use in regulating a variety of responses and development. For these and other reasons, there is a need for the present invention.